Milestones

The ancient city of Petra lies deep in a valley of Jordan

surrounded by steep, impassable sandstone walls and winding,

faulted gorges. Taken from the Latin word ‘petrae’, meaning

‘rock’, Petra’s sandstone is the oldest in a series of formations

underlain by pre-Cambrian granite with gneiss and schist. The

sandstone consists of coarse- to medium-grained quartz clasts, shot

through with an iron-and-manganese-rich matrix, and it varies in

colour from red to yellow to chocolate.

Geologists have identified the formation as the remnant of a large,

braided stream complex, buried by another white, friable sandstone

formation – a limestone layer caps high points throughout the region.

Petra was ‘re-discovered’ – Bedouins were living there – and

consequently revealed to the Western world in 1812 by 25-year-old

Swiss aristocrat-turned-explorer Johann Burckhardt, who had heard

whispers of some interesting ruins in the Valley of Moses (‘Ain Musa).

After following a stream through an ever-narrowing gorge, he came

across a towering mausoleum carved into the face of the sandstone

cliff. Further investigation revealed a Roman-style theatre, the

foundations of buildings, crumbled columns and paved streets –

indications that a large city had once stood here.

Although archaeological evidence indicates occupation in the valley since

7000 BCE (before the common era), it was the indigenous Arab residents,

the Nabataeans, and their Roman visitors who brought great power to

the region. And it was their craftsmen who worked the red-brown and

yellow sandstone walls of Petra into simple caves and elaborately-carved

tombs and structures, many exceeding 50 metres in height.

The Romans took over the desert metropolis at around 100 CE and

immediately set about installing a complex integrated system of hand-

carved stone flumes (some lined with ceramic pipes), reservoirs and

200 cisterns capable of supplying as much as 46,000 litres of water a

day to the settled valley. This intricate water management system

helped protect the city against erosion.

A devastating earthquake in 551 CE all but brought the city to ruin.

Approximately 800 carved cliff monuments survived, but the living city

of Petra died – trade routes had shifted sometime in the previous

century, and historians believe that the community lacked the wealth

to rebuild its central town structures after their destruction.

Without any maintenance, Petra’s water system crumbled. The

Nabataean dams and canals no longer diverted water flow away from

the tombs and town, making the sandstone that composes most of

Petra’s monuments and its Roman theatre once more susceptible to

accelerated weathering.

University of Arkansas geosciences professor Tom Paradise works with

the Petra National Trust, a Jordanian non-government organisation. He

has spent more than a decade studying the deterioration of the

sandstone and the architecture at Petra, seeking to determine the

effects of sandstone composition, climate, sunlight and tourism on the

disintegration of its structures.

Intrinsic (lithologic constituents, fractures) and extrinsic (climate,

human contact) weathering influences on the sandstones, show that

they are deteriorating at an accelerated rate due to natural and human-

induced stone decay processes.

“In particular, the crowds of people visiting Petra each year have left

unexpected marks on the ancient rocks and greatly accelerated the

natural processes at work,” he said.

Visitor impactIn 1980, the number of annual visitors to Petra was 100,000. By 1989,

largely thanks to Hollywood blockbuster Indiana Jones and the Last

Crusade, this number increased three-fold to 350,000. By 2000 at least

half a million people were traipsing all over the World Heritage site.

In January this year, the Jordan Times reported that political conflict in

the region had dramatically affected tourism in Jordan in 2006, with

the number of visitors to the ancient Nabataean city down by 8.6 per

cent to 359,366.

Preserving Petra the crumbling city of stoneby Tina Luton

Al-Khazneh tomb is Petra’s most famous attraction.

38 issue 11 www.discoveringstone.com.au

Preserving Petra the crumbling city of stoneby Tina Luton

Tourists inside Al-Khazneh dramatically increase relative humidity.

Bad news for resources-poor Jordan which relies heavily on its tourism

revenues – the largest drawcard of which is Petra – but good news for

the now crumbling city of stone.

One of the most popular attractions at Petra is the grand Al-Khazneh

tomb.

Elaborately carved with classical elements (pediment, columns,

entablature, etc.), it consists of a large staircase, two small exterior

flanking chambers and a primary chamber with three antechambers.

Over the years, Tom and his students have monitored how people in

the tour groups behave in the tomb, from where they put their hands

to where they place their feet – and they compare this to what has

disappeared.

While the sandstone’s finely-hewn detailing along the jambs, pilasters,

mantle and entablature remains relatively unaltered, most chamber

interior detailing has deteriorated. The four interior walls exhibit

original stone dressing marks. However, in the past decade many of

these dressed grooves have rapidly deteriorated from 0.5 to 2.5 metres

above the tomb floor.

Surface erosion was mapped in 1999 using laser levelling devices to

create a virtual netted surface model. It was found that across the 4 × 3

metre tomb wall surface, as much as 40 mm of sandstone had eroded,

in less than 50–100 years, as a result of visitors touching the stone.

“One whole chunk of wall of the Khazneh had lost half a cubic metre of

stone over a few years, because that’s where tour guides let people sit

while they talk about the history of the site,” Tom said.

“In fact, on the more than 100 occasions that Al-Khazneh was visited,

local police, tourists and tour guides were found leaning, touching,

rubbing and propping their feet against the chamber walls, producing

small, loose sand piles at the base of the measured wall,” he added.

The appearance of strange white deposits on the walls inside the

structure led Tom to take a sample. He assumed it was some kind of

salt deposit growing in flower-like structures on the surface. Lab tests,

Geosciences professor Tom Paradise.

The Roman theatre once seated up to 10,000 people but is now eroding under

the soft, gripping soles of tourists.

Petra attracts thousands of visitors each year.

www.discoveringstone.com.au issue 11 39

00 issue 11 www.discoveringstone.com.au

Milestones

however, showed the deposits to be stearic acid – or beef fat, a

component of cheap suntan oil once commonly available in the

Middle East.

“People would rest by leaning against the wall with sweating hands. This

is when you realise that what is happening at Petra is bad,” he said.

Increased humidity is one of the biggest problems. Environmental

monitoring of the interior of Al-Khazneh (conducted repeatedly since

1998) has revealed a very strong relationship between large numbers

of visitors and a subsequent rise in

relative humidity. Increased moisture

in restricted spaces can increase the

production of efflorescence (surface

salts) in-rock permeability and

moisture wicking, and cause a

general accelerated weathering and

surface recession of sandstone due to

particle disaggregation.

“The greatest increases in humidity

occur when visitor groups exceeding

25–30 people remain within the tomb

for more than five minutes. During

peak tourism season, 35 or more

people occupy Al-Khazneh almost all

day,” said Tom.

“With more than 30 people entering the tomb the humidity jumps

from 30 to 60 per cent, which is unacceptable in relation to

preservation of the sandstone.”

Possible solutions to decrease interior humidity fluctuations may

involve modification of interior microclimates (such as adding fans or

dehumidifiers) or the restriction of in-tomb visitor numbers at any one

time, with sufficient gaps between groups to permit the tomb

chambers to re-stabilise to a naturally lower humidity. In addition, a

solution to decrease wall abrasion by visitor contact may be to simply

restrict or prohibit direct access to the tomb interior walls.

Erosion of thE roman thEatrEAl-Khazneh is not the only structure at Petra to have suffered under the

weight of increasing tourism.

Tom has also conducted an extensive

study on the Roman-style theatre,

which may have seated up to 10,000

people.

Although it was carved before full

Roman occupation in 106 CE, it was

hewn out of the surrounding

sandstone cliffs – according to the

canon of the great Roman engineer

Marcus Pollio Vitruvius – using the

highest Roman engineering and

construction standards of the period.

These early requirements for theatre

and building construction were so

standardised that the level of the

original surfaces can be estimated

from the current receded surfaces.

Five hundred locations were examined across the theatre for intrinsic

factors, such as variations in rock composition and particle size, and

extrinsic influences including sunlight angle, lichen coverage, slope,

and surface temperature.

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“From 13 years of field research in Petra’s

Roman theatre, Anjar quarries and Al-

Khazneh tomb, we are able to better

understand the complex dynamics of

sandstone decay in Petra and in arid

regions and to develop conservation

strategies that will successfully protect

this unique architecture...”

Tom Paradise, geosciences professor.

The research found that sandstone recession is accelerated by insolation

(where temperatures exceed 50 °C), by carbonate content, and by wide

spacing of sandstone sand grains (high matrix-to-clast ratio). Rock

properties could also be interpreted on the basis of sandstone colour,

since paler calcium-rich and/or high-matrix sandstones are often softer

and more friable than darker coloured, iron-rich sandstones. It also

revealed that lithologic factors, such as iron oxides and silica in the

sandstone matrix, dramatically decrease weathering.

Simple observations supported these findings where the original

Roman stonemason tool marks were obvious atop iron-rich sandstone

(dark reddish), while large, weathered cavities had developed on the

rock areas that were iron poor (pinkish-beige to white).

It was also found that when matrix iron concentrations exceeded four

per cent (by weight), dressing marks appeared so fresh and deep that

little or no sandstone deterioration was apparent.

Inferential statistical analysis found that the iron and silica proportions

of the sandstone matrix explained 50 per cent of all deterioration in

the theatre.

The other 50 per cent could be put down to human traffic.

“Deterioration of the sandstone of the Roman theatre has recently

accelerated. When it was first examined for this research in 1990, at

least 15–20 per cent of the theatre displayed original stonemason

dressing marks made 2000 years ago. In 2001, only 5–10 per cent of

the marks were still evident,” explained Tom.

As such a dramatic change in the rate of decay could not be attributed

to intrinsic or climate changes, Tom surmised that it must be the result

of changes in tourism – not only to increased numbers but also to

changing fashion.

“Erosion has been enhanced by shoes with soft, gripping soles that

increase the relative friction between the visitors’ feet and the

sandstone. The accelerated surface recession is especially evident near

the orchestra, praecincterae (concentric walkways) and itinerae; parts

of the theatre most commonly visited by tourists and tour group

operators,” he said.

“Therefore, however sensitive visitors may be in their interaction with

monuments and landscape, visitor-accelerated weathering in Petra will

only decrease when tourism decreases, when shoe soles become less

abrasive, and/or when visitor access is restricted.”

Naturally, any kind of footwear that presses residue particles of

sandstone against the substrate will create additional abrasion.

One recommendation is to limit access to the orchestra and walkways,

with minimal access given to the main theatre where tourists – often

encouraged by tour guides – scramble across the sandstone seats to

the top of the theatre for a great view of Petra.

thE hiErarchy of wEathEringWhile weathering studies of sandstone architecture in arid

environments are relatively rare, research indicates that sandstone

principally weathers in two ways.

Since it is comprised of clasts (particles) within a matrix (binder),

either the clast fractures or dissolves and then falls out, or the matrix

fractures or dissolves to release the clast.

Both weathering types represent the processes of disaggregation that

produce loose sand as the by-product of sandstone deterioration, the

source of many of the sand dunes throughout the Near East and

North Africa.

Tom’s study established a previously unknown hierarchy of weathering

processes responsible for sandstone decay.

It was found that general rock composition (measured through

backscatter scanning electron microscope) was the most important

single influence (25 per cent), followed by the effects of iron

concentration (17 per cent) and climatic influences, such as sunlight

and moisture (12 per cent).

“These findings emphasise the importance of lithology in

understanding and predicting stone decay rates in arid regions,

specifically in Petra. Such a hierarchy is vital in grasping the

comparative controls on sandstone deterioration and the possible

priorities needed in conservation applications and research,” said Tom.

Other studies examined topography, aspect and moisture availability as

surrogates for insolation-induced weathering. These studies have

explained the importance of aspect, heating-cooling cycles, and

moisture availability in the acceleration of surface erosion and/or

Milestones

Removed lichens and rock varnish due to tourists resting and touching. Petra sandstones are banded in spectacular colours.

42 issue 11 www.discoveringstone.com.au

weathering feature development, reinforcing the notion that received

solar radiation is important in the acceleration of weathering.

Among the sandstone cliffs of Petra are ancient Nabataean quarries,

including those high above at Anjar where the Petra sandstone was

sourced, and a number of unique carved cubic sculptures called the

Djin Blocks.

The surfaces of the Anjar quarry and the blocks were studied as many

of these hewn surfaces face different aspects, making them ideal for

observing the effects of sunlight on stone deterioration.

The Nabataeans prepared the quarry surfaces by chiselling and dressing

them into distinctive herringbone patterns, a style not used by Roman

stonemasons.

At Anjar, the largest recession features were found on south-west and

south-east faces, supporting the importance of the tandem role of

heating/cooling and wetting/drying cycles.

Weathering features (i.e. tafoni, cavities) on the dressed quarry faces

were measured, and a number of relationships were divulged.

Northern-facing surfaces showed the least erosion with 90 per cent of

the original stone-dressing visible and no recessional features

exceeding two centimetres in any dimension. The discrete and

measured features were then mapped where their dimensional values

(width, depth) were related to the quarry wall aspects.

The relatively minor weathering observed on northern faces can be

attributed to decreased weathering from lichen overgrowth, since

lichens are rarely found on other surfaces, and to the fact that less

sunlight produces fewer wetting and drying cycles.

“Even though it has been widely accepted that lichen attachment can

accelerate substrate weathering through rhizinal penetration and oxalic

acid production, in Petra it was found that lichen overgrowth acts as a

sandstone surface–consolidating agent, which decreases overall surface

recession,” revealed Tom.

Southern faces displayed 40 per cent of the original dressing with few

recessional features larger than 15 cm in any dimension. Western to

south-western and eastern to south-eastern faces, however, displayed

the greatest amount of deterioration and recession, with less than 10

per cent of the original Nabataean stone dressing remaining and

cavities exceeding 20 cm across.

This may be attributed to the optimal daily and yearly climatic regime

experienced by these quarry faces, where temperatures are hot enough

to expand the sandstone, causing disaggregation, and repeatedly moist

Milestones

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Photographs of the hewn finial on al-Deir (the monastery) shot in 1928 (left) and 2005, show little natural deterioration in the sandstone over the past 75 years.

conditions cycle the rock between hot and cool, thawed and frozen,

wet and dry – cycles known to accelerate rock weathering.

A similar distribution of tafoni related to aspect was observed on the

Djin Blocks. These discoveries reinforce the conventional notion that the

deterioration of sandstone is greatly accelerated from increased heating

and cooling or wetting and drying cycles. However, it is now believed

to be faster and more destructive than previously understood.

“In Petra, this research demonstrates the delicate balance of

weathering between climate and lichen overgrowth on stone

architecture,” observed Tom.

In many countries, such as Italy, that actively work to protect their

architectural masterpieces, conservationists once peeled off the lichen

growing on historic buildings but they now recognise that in some

settings lichens act as a natural seal that can protect rocks from erosion

and weathering.

Methods of preserving architectural treasures have certainly changed

over the past century, as archaeologists, geoscientists and historians

adjust their approach to conservation and restoration, but in Petra one

proposed method to preserve the rock faces would have actually

destroyed them.

“Researchers wanted to apply a type of silica gel to the rocks,”

explained Tom. “It’s a great product for statues, which can be dipped

into the sealant and allowed to soak it up over days. If it’s done

perfectly and every pore is sealed, it works great, but if it has one little

crack in it, it concentrates moisture, which accelerates weathering in

the long run and may cause spalling (where outer rock layers peel

after freezing and thawing),” he said.

After more than 100 years of continual excavation at Petra, only a

small percentage of the city has been revealed. Despite this, hundreds

of thousands of tourists continue to flock to the site each year.

Unfortunately, they bring with them a legacy that threatens to reduce

the ancient city of stone to the shifting desert sand that surrounds it.

Something that Tom, who is returning to Petra in May to continue his

weathering research and people-watching, finds disturbing – although

he is optimistic.

“From 13 years of field research in Petra’s Roman theatre, Anjar

quarries and Al-Khazneh tomb, we are able to better understand the

complex dynamics of sandstone decay in Petra and in arid regions and

to develop conservation strategies that will successfully protect this

unique architecture from accelerated deterioration due to increasing

regional and global tourism,” said Tom.

As an interesting footnote, Tom’s current research also indicates that a

large flood, or series of flash floods, shut down the main colonnaded

street of Petra and its ‘shopping mall’ in the third century. Evidence of

flooding includes deep deposits throughout the interiors of the Roman

street shops excavated in the mid-1990s.

“Flood deposits of up to one foot (about 30 cm) in depth were

excavated in the 1990s a good 10-15 feet (four metres) above the

main stream channel of Wadi Musa. These would have been disastrous

waves and since the depositional particles were sands and not silts,

this implies a high-energy rushing water state. Also, the sand was

blond in colour not like the Umm Ishrin sandstone in the valley floor,

but from the Disi sandstone outside the valley. These floods had to be

catastrophic and when we return to Petra in May we will collect more

data. When we prove it – and I am confident that we will – it will add a

new chapter to Petra history,” Tom concluded.

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